Our intention with Sandbox is to take a powerful game engine (UE4 right now) and build a hotloading C# layer on top of it. The idea being that you will never have to touch C++, you never have to wait for things to compile, you never have to close the game while developing.

We're hoping that what we're making is engine agnostic. Our goal was to build this on top of Source 2, but that seems unlikely now - as it's doubtful that would have any actual benefit over what we're currently doing.